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1. Field of the Invention
The invention relates to the field of optical communication systems, and in particular, to a Metropolitan Area Network (MAN) using low insertion loss Optical Add-Drop Multiplexers (O-ADMs).
2. Description of the Prior Art
Fiber optic networks vary in size to accommodate different communication needs. Wide Area Networks (WANs) span the nation providing communications over long distances. Local Area Networks (LANs), in contrast, provide communications over short distances, such as in a building. In between WANs and LANs are Metropolitan Area Networks (MANs). MANs are smaller than WANs and larger than LANS, and typically range in size from approximately 25 km to 100 km. MANs are typically used for communications on a campus or in a city.
WANs use optical fiber amplifiers to boost optical signals transmitted over the network because of the expansive distances covered by the networks. MANs typically do not use optical fiber amplifiers in an attempt to keep costs and design complexity at a minimum. The smaller size and absence of optical fiber amplifiers could be features that distinguish MANs from WANs. Without optical fiber amplifiers, MANs are limited in how large they can grow.
Fiber optic networks, including MANs, often utilize multiplexing technologies to increase the volume of traffic upon the network. One such multiplexing technology is Wavelength Division Multiplexing (WDM). WDM is used to pass multiple data channels over one or more wavelengths of light simultaneously over a single fiber. As an optical signal is transmitted over the fiber, wavelengths can be dropped or added at nodes set out in the network. The nodes typically use Optical Add-Drop Multiplexers (O-ADMs) to add wavelengths to, and drop wavelengths from, the optical signal. Typically, a node is assigned a wavelength so that each node in the system drops and adds different wavelengths.
FIG. 1 shows a Metropolitan Area Network (MAN) 100 in the prior art. MAN 100 is comprised of a fiber 110, a central node 112, and O-ADMs 121-126. Each end of fiber 110 is coupled to the central node 112 to form a ring. O-ADMs 121-126 are coupled to fiber 110 in series. Central node 112 is connected to a first system (not shown) and configured to transmit an optical signal comprised of wavelengths xcex1-xcexn over fiber 110, transmit signals to the first system, and receive signals from the first system. O-ADMs 121-126 are configured to drop wavelengths from, and add wavelengths to, the optical signal.
In operation, central node 112 transmits the optical signal comprised of wavelengths xcex1-xcexn over fiber 110. O-ADM 121 receives the optical signal from central node 112. O-ADM 121 drops a wavelength xcex1 from the optical signal and transfers xcex1 to a second system (not shown). O-ADM 121 also receives xcex1 from the second system and adds xcex1 back to the optical signal. O-ADM 121 transfers the optical signal to O-ADM 122. O-ADM 122 drops a wavelength xcex2 from the optical signal and transfers xcex2 to a third system (not shown). O-ADM 122 also receives xcex2 from the third system and adds xcex2 back to the optical signal. O-ADM 122 transfers the optical signal to O-ADM 123. The same operation takes place over O-ADMs 123-126. Central node 112 receives the optical signal comprised of wavelengths xcex1-xcexn from O-ADM 126. Two common O-ADMs used in the art are single fiber grating O-ADMs and dielectric film filter type O-ADMs.
FIG. 2 shows a single fiber grating O-ADM 200 commonly used in MANs in the prior art. O-ADM 200 is comprised of a first optical circulator 220 coupled to a second optical circulator 222 by a fiber 214. Between optical circulators 220 and 222, a Bragg grating 230 is written into fiber 214. The Bragg grating 230, which is based on the Bragg effect,is a periodic perturbation of the effective refractive index of fiber 214, wherein fiber 214 is photo-sensitive. The Bragg grating 230 is configured to reflect a narrow or broad range of wavelengths of light while passing all other wavelengths. The Bragg grating 230 is written into fiber 214 with a laser beam of Ultra-Violet (LV) light. The UV light permanently changes the refractive index of fiber 214.
In operation, optical circulator 220 receives an optical signal comprised of wavelengths xcex1-xcexn over a fiber 210. The optical signal passes through optical circulator 220 to the Bragg grating 230. The Bragg grating 230 drops a wavelength xcex1 from the optical signal by reflecting xcex1 back to optical circulator 220. Optical circulator 220 prevents xcex1 from propagating over fiber 210 and transfers xcex1 over a fiber 212. The optical signal comprised of wavelengths xcex2-xcexn passes through the Bragg grating 230. Optical circulator 222 receives thepoptical signal comprised of wavelengths xcex2-xcexn over fiber 214 and xcex1 over a fiber 216. Optical circulator 222 adds xcex1 back to the optical signal. O-ADM 200 transfers the optical signal comprised of wavelengths xcex1-xcexn over a fiber 218. Optical circulators 220 and 222 typically have an insertion loss between 0.8 and 1.0 dB. Thus, the insertion loss of O-ADM 200 is typically above 1.6 dB. The strength of the optical signal is appreciably diminished by the insertion loss of O-ADM 200.
FIG. 3 shows a dielectric film filter type O-ADM 300 also commonly used in MANs in the prior art. O-ADM 300 is comprised of a first dielectric WDM add-drop filter 320 coupled to a second dielectric WDM add-drop filter 322 by a fiber 314.
In operation, dielectric WDM add-drop filter 320 receives an optical signal comprised of wavelengths xcex1-xcexn over a fiber 310. Dielectric WDM adddrop filter 320 drops a wavelength xcex1 from the optical signal by filtering xcex1 and transfers xcex1 over a fiber 312. The optical signal comprised of wavelengths xcex2-xcexn passes through dielectric WDM add-drop filter 320 and over fiber 314. Dielectric WDM add-drop filter 322 receives the optical signal comprised of wavelengths xcex2-xcexn over fiber 314 and xcex1 over a fiber 316. Dielectric WDM add-drop filter 322 adds xcex1 back to the optical signal. O-ADM 300 transfers the optical signal comprised of wavelengths xcex1-xcexn over a fiber 318. Dielectric WDM add-drop filters 320 and 322 typically have an insertion loss between 0.8 and 1.0 dB. Thus, the insertion loss of O-ADM 300 is typically above 1.6 dB. The strength of the optical signal is appreciably diminished by the insertion loss of O-ADM 300.
Fused fiber O-ADMs have been disclosed that have a lower insertion loss than O-ADM 200 in FIG. 2 and O-ADM 300 in FIG. 3. FIG. 4 shows a fused fiber O-ADM 400. O-ADM 400 is comprised of a first fiber 410 coupled to a second fiber 412. A portion of first fiber 410 is fused to a portion of second fiber 412 to form a fused region 414. The fused region 414 has a first side 421 and a second side 422. A Bragg grating 416 is written into the fused region 414 as discussed in FIG. 2. First fiber 410 is configured to couple to a fiber optic network:(not shown) carrying optical signals. Second fiber 412 is configured to couple to a system (not shown, wherein the system is configured to transmit and receive a wavelength xcex.
In operation, O-ADM 400 receives an optical signal comprised of wavelengths xcex1-xcexn over first fiber 410 on the first side 421 of the fused region 414. The optical signal travels into the fused region 414 and the Bragg grating 416 drops a wavelength xcex1 from the optical signal by reflecting xcex1 back over the second fiber 412. Wavelength xcex1 does not reflect back over the first fiber 410 or pass through the Bragg grating 416. The optical signal comprised of wavelengths xcex2-xcexn passes through the Bragg grating 416 and over first fiber 410 on the second side 422 of the fused region 414. On the second side 422 of the fused region, O-ADM 400 receives xcex1 over second fiber 412. Wavelength xcex1 travels into the fused region 414 and the Bragg grating 416 reflects xcex1 back over first fiber 410. The Bragg grating 416 adds xcex1 back to the optical signal. O-ADM 400 transfers the optical signal comprised of wavelengths xcex1-xcexn over fiber 410. The O-ADM 400 typically has an insertion loss of less than 0.1 dB.
A problem with MAN 100 is the current O-ADMs being used to drop and add wavelengths have high insertion losses, such as O-ADMs 200 and 300. Each O-ADM that is added to MAN 100 that has an insertion loss of above 1.6 dB will seriously degrade the optical signal. Because MANs typically do not utilize optical fiber amplifiers, the size of MAN 100 is limited. Also, the number of nodes using O-ADMs to drop and add wavelengths is limited.
A Metropolitan Area Network (MAN) solves the above problems by utilizing low insertion loss O-ADMs to drop wavelengths from, and add wavelengths to, an optical signal. The MAN is comprised of a central node, a fiber, a first Optical Add-Drop Multiplexer (O-ADM), a second O-ADM, and a-third O-ADM. Each end of the fiber is coupled to the central node forming a ring, and the first, second, and third O-ADMs couple to the fiber in series. The first, second, and third O-ADMs each have an insertion loss that is less than approximately 1.00 dB. The MAN is able to grow much larger than prior MANs because of low insertion loss O-ADMs.
In one embodiment, the first, second, and third O-ADMs are fused fiber OADMs. A fused fiber O-ADM is comprised of a first fiber coupled to a second fiber. A portion of the first fiber is fused to a portion of the second fiber to form a fused region. A fiber grating is written into the fused region. The fiber grating reflects wavelengths from optical signals and the fiber grating can reflect different wavelengths depending on how it is written into the fused region.
The invention is much more efficient than current MANs. The O-ADMs used in the current networks typically have insertion losses above 1.6 dB, while the invention uses O-ADMs with insertion losses of less than approximately 0.1 dB. Therefore, when optical fiber amplifiers are not implemented, the fiber optic network can be many times larger than the current networks.